The present invention relates to a process for the eloctrolytic pickling of chromium-containing stainless steel, in which initially pickling takes place in an aqueous Na.sub.2 SO.sub.4 - solution and thereafter in acid, preferably mixed acid, optionally without electric current. Such a process as described in AT-PS No. 252.685, wherein hydro-fluoric acid plus nitric acid serve as the mixed acid. In the first step the millscale is removed; whilst in the second step the chromium-depleted layer underneath the millscale, formed during annealing, is dissolved off. That process has gained worldwide recognition, e.g. for the pickling of stainless steel strip, but suffers from the disadvantage, that the Cr in the millscale is oxidised by the current to CrO.sub.4.sup.2-, whilst Fe, when dissolved, is precipitated immediately as Fe(OH).sub.3. The CrO.sub.4.sup.2- which is formed, remains in solution and is only removed during de-sludging, respectively together with the stainless steel strip and is only then detoxified by the reducing agent. For that purpose, primarily the reduction with an aqueous FeSo.sub.4 -solution in the pH-range of 0-2 respectively 7-8 have found acceptance. In both cases it is necessary subsequently to neutralise again, in order to precipitate all metal ions in the solution in the form of hydroxide. Moreover, in the event of a sudden availability of very large quantities of CrO.sub.4.sup.2- -containing solutions, there always exists a risk of a breakthrough of CrO.sub.4.sup.2-, which may then enter into the effluents. A further drawback of the process is that only a fraction of the CrO.sub.4.sup.2- formed is removed from the aqueous solution by the Na.sub.2 SO.sub.4, whilst the balance increases the concentration in the solution and results in increased attack on plastics pipelines and pumps.
In the publication "Chemical Abstracts", Vol. 87, No. 14, Oct. 3rd, 1977, on page 396, Abstract No. 108322s, the formation of CrO.sub.4 "respectively Cr.sub.2 O.sub.7 " is indeed described, but no solution to this problem is offered.